[PATCH v6] MIPS: Reject branch absolute relocs for PIC for linking

[Hans-Peter Nilsson]

A random comment:

On Wed, 21 Feb 2024, YunQiang Su wrote:

> The asm code like:
> 	b	8
> will emit absolute relocs like:
> 	R_MIPS_PC16	*ABS*
> 
> If they are included into PIC shared objects or PIE executables,
> the branch target will be like 0x12340000, which will make the
> programs crash.
> ---
>  bfd/elfxx-mips.c                                | 9 +++++++++
>  ld/testsuite/ld-mips-elf/mips-elf.exp           | 1 +
>  ld/testsuite/ld-mips-elf/pic-reject-abs-reloc.d | 5 +++++
>  ld/testsuite/ld-mips-elf/pic-reject-abs-reloc.s | 2 ++
>  4 files changed, 17 insertions(+)
>  create mode 100644 ld/testsuite/ld-mips-elf/pic-reject-abs-reloc.d
>  create mode 100644 ld/testsuite/ld-mips-elf/pic-reject-abs-reloc.s
> 
> diff --git a/bfd/elfxx-mips.c b/bfd/elfxx-mips.c
> index 69dd71419ff..9542250dec4 100644
> --- a/bfd/elfxx-mips.c
> +++ b/bfd/elfxx-mips.c
> @@ -9258,6 +9258,15 @@ _bfd_mips_elf_check_relocs (bfd *abfd, struct bfd_link_info *info,
>  		   (h) ? h->root.root.string : "a local symbol");
>  	      break;
>  	    default:
> +	      if (branch_reloc_p (r_type) && r_symndx == STN_UNDEF)
> +		{
> +		  howto = MIPS_ELF_RTYPE_TO_HOWTO (abfd, r_type, NEWABI_P (abfd));
> +		  info->callbacks->einfo
> +		    /* xgettext:c-format */
> +		    (_("%X%H: relocation %s against `*ABS*' cannot be used"

There's no "*ABS*" in the source and IMHO that'd look confusing 
to innocent users.  How about "...against an absolute value"?  
Or "...against an absolute value or absolute symbol"?  Perhaps 
the latter is a bit too wordy, but also more complete.

> +		       " when making a PIC/PIE object\n"),
> +		     abfd, sec, rel->r_offset, howto->name);
> +		}
>  	      break;

brgds, H-P